Background of the Invention
1. Field of the Invention
[0001] This invention relates to a process for preparing a ceramic porous body, specifically
to a process for preparing a ceramic porous body whose process uses the reaction sintering
of a metal- ceramic mixture molded by a slurry casting method.
2. Prior Art
[0002] For the preparation of a sintered ceramic product the generally adopted conventional
method is one in which a green molded product is prepared by compression molding ceramic
grains by a press and then heat-sintering the product. However, this method has problems
in the high preparation cost and limitation of the shape and size due to the use of
a mold. To form a complicated shape, a method of mixing a thermoplastic resin with
ceramic grains and subjecting the mixture to injection molding has been conducted.
This method has problems such as the segregation of the components in the molding
process, and the complexity of the degreasing process, in addition to the limitation
on dimensions. As a more simple method to form a product of a complicated shape, one
is used in which ceramic grains are suspended in a solvent to prepare a slip, the
suspension is cast in a mold of porous material such as gypsum, with the solvent being
absorbed by the porous material to form a green compact. However, this method has
problems in that only grains having small diameters can be used and in that the cost
for preparation is high because the removal of water is time-consuming and this results
in requiring many kinds of molds.
[0003] In addition to the above, since the sintering temperature of ceramic grains is as
high as 0.5-0.8 times their melting point or the decomposition temperature a high-temperature
oven is necessary for sintering the ceramic grains and thereby the preparation cost
gets higher.
[0004] A method has been conducted by Yoshimura (J. Materials Sci. Let., 9 (1990) 322-325)
in which a metal is melted in a gas atmosphere at a high temperature and gas-reacted
metal compound grains are continuously precipitated on the surface of the molten metal.
However, by this method the molten metal is consumed and becomes hollow, and the resulting
gas-reacted metal compound takes an irregular shape, unable to form a stable shape.
Because of this fact, a method has been conducted by M. S. Newkirk (J. Mater. Res.
1(1)-(1986),81-89) in which the voids among grains or a molded porous product placed
on a molten metal are impregnated with the molten metal to form a shaped product.
This does not necessarily give a product of a stable shape since this method is accompanied
by the movement of the molten metal.
[0005] Also, an attempt has been made to heat metal grains or a molded product thereof in
a gas atmosphere to produce a gas-reacted metal compound. However, this method has
a drawback in that a gas-combined product is not readily produced since sintering
among metals takes place first. To resolve this drawback a method has been conducted
in which gas-reacted metal compound grains are interposed as an admixture among metal
grains. However, the diameters of ceramic grains are smaller than those of metal grains
and the obtained product is a grain compact. Thus, as Matsuo et al. reported (Yogyo
Kyokaishi (J. Ceram. Asoc. Japan), 73(2)(1965),82-86), since a molten metal is bled
when sintering is done, no great amount of metal grains can be included in the product.
[0006] Since the sintering of the usual ceramics is accompanied by a change in the shape
and by a shrinkage of about 10-30%, the size of the sintered product cannot be controlled.
Thus, a method is used where a low temperature-calcinated product that is not completely
sintered is produced and that calcinated product is roughly processed and then completely
sintered. Accordingly, the method requires complicated processes and it has been difficult
to produce a porous sintered body of a large size and complicated shape with good
air permeability.
Summary of the Invention
[0007] The present invention has been conceived to resolve said problems. The object of
the present invention is to provide a process for readily preparing at a low cost
a sintered porous product with a large size and a complicated shape, said product
having a good porosity and mechanical properties and being suitable, from the aspect
of a filtering function, for the filtration of gas, i.e., compressed air, various
kinds of gases, or steam, and the filtration of a liquid, i.e., water, an aqueous
solution, an electrolite, oils, a molten product of a synthetic resin, or a molten
metal; from the aspect of a separation or concentration function, for the separation
or concentration of gas, i.e., a mixed gas or a gas equivalent, or for the separation
or concentration of a liquid, for instance, by the reverse osmosis method, etc.; from
the aspect of the breathabil- ity through communicating pores, for devices for the
blowing of gas, i.e., a base for transporting grains and granular material, an aeration
(an air diffuser pipe), an air roll, or air bearing, for relieving gas, i.e. for a
gas passage, a vent hole, a vacuum leakage valve, a vacuum chuck, or a breathable
or durable mold, for a spouting liquid, i.e. for a fluid injection electrode, a surface
plate for spouting an abrasive grain mixture slurry; from the aspect of shock absorption,
for a sound-deadening material, i.e. a sound-deadening or sound-adsorbing material
for a pneumatic machine, for a cushioning material, i.e. a material for preventing
the pulsation of compressed gas, or a damping material, for a compressed elastomer,
i.e., a shock absorber for a sealing material; from the aspect of a high specific
surface, for a material for chemical reaction, i.e. a catalyst, a carrier for a catalyst,
a sensor, or a battery electrode material, or for a material for physical action,
i.e. an adsorbing material, for a heat transferring material, i.e. antiflashing material,
a heat-exchanger element, or a heating element; from the aspect of capillarity, for
transferring a liquid, i.e. for a wick or a feedwater roller, for a feeding liquid,
i.e. for a sweat-cooling material, a humidifier, a mist lubricator, an ink roller,
or a bearing; from the aspect of flow control, for controlling a flow or a flow rate,
i.e., a dispensing flow plate or a dispersing flow plate, or for controlling a gas
boundary layer.
[0008] To achieve the object the present invention adopts a method in which an aggregate
consisting of metal grains and ceramic grains, to which fibers may be optionally added,
is mixed with a binder with stirring to form a slurry mixture, which is then cast
in a mold, and solidified to obtain a hardened product, the product is dried, and
after drying, is heated in an ambient atmosphere, and sintered.
[0009] When sinterable metal grains and ceramic grains are used, or they are used together
with fibers, it is most preferred to use metal grains that can be reaction-sintered
with the atmosphere or/and ceramic grains or/and fibers, and to heat them to such
a temperature that causes reaction sintering.
Detailed Description
[0010] The present invention will now be explained in detail.
[0011]
Figure 1 shows a molded product. It consists of a mixture of metal grains 1 and added
ceramic grains 2.
Figure 2 shows a ceramic porous body sintered in a gaseous atmosphere. It consists
of a structure of metal grains 1 bonded to added ceramic grains 2. This is the case
in which the gaseous atmosphere is inactive to, or metal grains are not chemically
combinable with, the atmosphere.
Figures 3, 4, and 5 show examples of the ceramic products obtained by the present
invention.
Figure 3 shows a ceramic porous body that is obtained by sintering in a gaseous atmosphere.
It consists of a structure of unreacted metal grains 1, added ceramic grains 2, and
compound grains 3 obtained by chemically combining metal powders with an atmospheric
gas, bonded to one another.
Figure 4 shows a ceramic porous body sintered in a gaseous atmosphere. It consists
of a structure of added ceramic grains 2 bonded to compound grains 3 obtained by chemically
combining metal grains with an atmospheric gas. In this case, if the compound grains
3 that are chemically combined with the atmospheric gas are the same compound as the
added ceramic grains 2, they will become integrated with each other. If they are different,
they will form a mixed structure.
Figure 5 shows a ceramic porous body obtained by sintering in an atmospheric gas.
It consists of compound grains 3 obtained by chemically combining metal grains with
the atmospheric gas, or compound grains 4 obtained by chemically combining metal grains
with the decomposed gas of added ceramic grains, added ceramic grains 2, and metal
5 produced by the decomposition of added ceramic grains, bonded to one another.
[0012] The process for preparing a ceramic porous body by the present invention is as shown
in Figure 6. To obtain a ceramic porous body that is aimed at by the present invention,
metal grains are first mixed with ceramic grains. Fibers, if used, are added to metal
grains and ceramic grains, and then a binder is added thereto, and the thus-added
mixture is thoroughly mixed with stirring to give the mixture slurry 6. The amount
of metal grains to be used is selected from the range of above 0 to 100% by volume
of the total amount of metal grains and ceramic grains. The amount of ceramic grains
to be used is selected from the range of 0 to below 100% by volume of the total amount
of the metal grains and ceramic grains. The amount of fibers, when used, is below
50% by volume of the total amount of metal grains and ceramic grains.
[0013] The metal grains are elements that produce ceramics by chemically combining with
gases. They include, for example, Al, Si, B, Be, Ti, Cr, Ta, Te, Pb, Sn, Zn, and alloys
thereof, among which one or more kinds are selected to be used.
[0014] The ceramic grains are those having a high fire resistance and a small deformation
ratio at high temperatures, and are obtained by chemically combining metal grains
with gas. They include, for example, oxides such as alumina, glass, mulite, barium
titanate, zinc oxide, copper oxide, and tin oxide, and nitrides such as aluminum nitride,
silicon nitride, titanium nitride, and zirconium nitride, carbides such as titanium
carbide, silicon carbide, and boron carbide, and borides such as titanium boride and
zirconium boride, among which one or more kinds are selected to be used.
[0015] The fibers may be long fibers, but they are apt to produce fiber balls. Thus, short
fibers having an aspect ratio of 50 or less are practical. For the short fibers substances
obtained by cutting fine lines, and whiskers can be properly used. It is usually proper
to add fibers to a mixture of metal grains and ceramic grains in an amount of below
50% by volume based on the total amount of metal grains and ceramic grains. The addition
of fibers improves dispersion of a binder and a mixture of metal grains and ceramic
grains, and thereby the occurrence of cracks on drying and sintering is prevented.
From the aspect of moldability and the functional improvement of fibers, it may also
be proper to use the material of the same type or quality as that of the metal grains
and ceramic grains. It is also proper to use fiber reinforcement.
[0016] For the binder, self-curing liquid binders for molding sand such as a water-soluble
phenolic resin, specifically, a liquid of hydrolyzed ethyl silicate, may preferably
be used. The first reason for this is that the catalytic action of hydrolyzed ethyl
silicate causes drastic gelling and the binder has such properties that it changes
from a fluidized state to a non-fluidized one. That is, when a self-curing liquid
binder is used, by adjusting the added amount of the hydrolyzed ethyl silicate curing
catalyst, the slurry mixture obtained by mixing the hydrolyzed product of ethyl silicate
with metal grains and ceramic grains can have a flowability sufficient to be easily
cast in a mold and can be rapidly solidified after molding to such a strength that
the molded product is readily separated from the mold. The use of the binder improves
the productivity without the need for a mold having a high strength or an expensive
press.
[0017] The second reason is that since the gelled product of the hydrolyzed ethyl silicate
consists of 20% silica and 80% ethyl alcohol, when a product molded and cured, with
the gelled product as a binder, is dried, the ethyl alcohol evaporates and the voids
remaining after evaporation result in fine communication pores, contributing to the
formation of pores in a sintered product.
[0018] Generally the amount of the binder may be properly selected within the range of 10
to 80 wt% of the total amount of metal grains and ceramic grains, and if used, fibers,
depending on the grain diameter distribution. The amount should be as small as possible.
The lowest amount used should be such that the slurry mixture does not flow without
the application of vibrations. However, from the aspect of workability, to make the
flowability of the mixture better, a higher amount is desirable. Since a high amount
of the binder often causes the segregation of the components, thereby causing malformation,
and a large amount of the binder should be removed, it is costwise undesirable. Accordingly,
the highest limit of the amount should be lower than the amount at which the binder
begins to separate from the mixed grains of the metal and ceramic. When a material
is prepared, the binder may be colloidal silica, carbon dioxide-cured water-glass,
or metal silicon-added water-glass, instead of the hydrolyzed ethyl silicate, and
if necessary an evanescent organic compound may be added.
[0019] The slurry mixture 6 obtained as shown above is cast in a flask 7 and is cured. In
this process a shape conversion may also be conducted by setting a model or an actual
product 8 in the flask. Then the thus-obtained product is placed in an oven to be
heated and sintered in a gas atmosphere.
[0020] Due to the cast molding the shape and size of the obtained product allows a high
degree of free choice. Thus not only a flat product, but also a tubular product, a
product having curved surfaces, and a product that has a three-dimensional structure,
can be freely formed. The cast molding can be, depending on the kind of binder, any
of vacuum knead cast molding, vibration cast molding, etc., but when a large amount
of the binder is used, gravity cast molding may also be adopted. In this case it is
more effective to pressurize a slurry mixture in a mold with a punch, because by pressurization
a surplus binder in the material, and air bubbles included in a solvent, can be flashed
and removed from the system, and a molded product with minimum irregular shapes can
be obtained. Also, since metal grains and ceramic grains are in greater contact, accelerated
sintering can be expected. However, excessive pressurization is not desirable since
a plastic deformation of the grains is caused and this requires an increase in the
rigidity and size of a pressurization machine. Accordingly, generally the applied
pressure may preferably be as low as a surface pressure of 60 Mpa or lower, and the
lowest limit of the pressure may be about 0.3 Mpa. A fluid pressure molding with such
a low pressure makes it possible to prepare a molded product having less defects at
a low cost by using a machine with a simple structure. The thus-obtained molded product
is released from the mold as shown in Figure 7, and then the released product is air-dried
for 1 to 48 hours to prevent cracking and the occurrence of deformation. If necessary,
that is, for example, when a binder contains an evaporating component, primary calcination
may be conducted, in place of or in addition to air drying, by a method in which the
molded product 9 is subjected to direct ignition or vacuum drying.
[0021] The thus-obtained molded product 9 is then sintered in a gaseous atmosphere. This
gaseous atmosphere may be an oxidizing, nitriding, or silicifying one. The oxidizing
atmosphere may be air or oxygen-added air. The nitriding atmosphere may be nitrogen
gas or ammonia gas. The silicifying atmosphere may be a gas of heated silicon or a
gas of heated silicon suboxide. Although the sintering conditions depend on the compounding
ratio of the molding materials and the average grain sizes of metal powders and ceramic
powders and the kinds of casting molds, generally the sintering temperature may preferably
be the melting point of the added metal or higher, and lower than the melting point
or the decomposition point of the added ceramic grains, and desirably about 1673K
or lower. The sintering period may be 1-50 hours. The reason that the upper limit
of the sintering temperature is made to be lower than that for sintering ceramics
is that sintering may be conducted at a low temperature so that an expensive sintering
oven can be avoided. In this process making a ceramic of metal grains by gaseous reaction
and sintering the ceramic proceed simultaneously to provide a sintered product.
Example 1
[0022] Aluminum grains having an average grain diameter of 20 µm were used for the metal
grains and alumina grains (average grain diameter of two levels: 4 µm (alumina A42-6)
and 44 µm (alumina A-12), both are prepared by Showa Renko K.K.) were used for the
ceramic grains in various compounding ratios, and both were kneaded. Then hydrolyzed
ethyl silicate was added as the binder to form a slurry, the thus-obtained slurry
was kneaded by vibrations, and the kneaded product was subjected to vacuum degassing.
The thus-obtained product was cast in an aluminum mold to form a specimen of & 10
x 20mm. After the alcohol was removed by air drying, sintering was conducted in a
gaseous atmosphere for 6 hours. In this Example, vibration slurry casting, which is
one of the non-pressure molding methods, was used for molding. This method is characterised
in that it makes possible charging grains into the mold without the need to use a
durable mold, and in that an air-hardening inorganic binder (silicate sol) is used
for a solvent, and thus a relatively uniform molded product can be easily obtained.
The amount of the binder necessary for molding changes as shown in Figure 8, depending
on the grain size distribution and compounding ratio of the molding materials. According
to the differential thermogravimetric analysis of the molded product, weight increases
due to oxidization were shown at a temperature of about 1123K or higher. Thus, sintering
was conducted at 1123K (900 °C) and 1573K (1300
°C). It is recognized that, as shown in Figure 9, the compressive strength of the sintered
product increases as the amount of the aluminum increases and that dimensional shrinkage
gets smaller as the expansion by oxidation reaction gets larger. In the use of alumina
having large grains the dimensional change tends to be small. The reason appears to
be that if the average diameter of the grains used is larger, a greater amount of
a binder is necessary, and thus the amount of silica remaining between grains gets
larger, such that the intergranular separation can be completely maintained to enable
oxidation progress.
[0023] Although by the oxdization and sintering of metal aluminum grains a certain amount
of aluminum remains, the direct oxidation of aluminum can produce a porous alumina
which has little dimensional change at a low temperature.
Example 2
[0024] An alumina having an average grain diameter of 20 µm and aluminum-12% silicon having
an average grain diameter of 25 µm were used for metal grains, and a mulite having
an average grain diameter of 29 µm was used for ceramic grains in various compounding
ratios, and both were kneaded. Then hydrolyzed ethyl silicate was added as the binder
to the kneaded product. The product was then kneaded by vibrations to form a slurry
and was vacuum-degassed, and then cast in an aluminum mold to form a specimen having
& 10 x 20mm. The alcohol was removed from the specimen by air drying and the specimen
was then sintered in a gaseous atmosphere for 6 hours. Due to the similarity in the
grain size distribution of aluminum grains or aluminum-12% silicon grains and that
of alumina grains as shown in Figure 10, the amount of the binder was almost constant.
The porosity of the molded product hardly changed when the sintering temperatures
were 823K and 873K, which are close to the melting point of aluminum, as shown in
Figure 11. However, when the sintering temperature was raised to 1173K (550 ° C) and
1573K (600
° C) it was observed that the porosity decreased and that the compressive strength got
larger. It was found that this tendency is eminent when the amount of the aluminum
is 28 vol.% or more, and this tendency is greater if the amount of aluminum is greater.
Where the amount of aluminum was 78 vol.% or more, a sweating phenomenon, that is,
the flowage of molten aluminum onto the surface of the specimen, was observed. When
aluminum-12% silicon grains were used, sweating at a relatively low temperature, compared
to aluminum, was observed. The sweating seems to depend on the amount of the molten
eutectic liquid. As shown in Figure 12, the dimensional change in aluminum-alumina-based
molded products obtained in this Example is within 0.1 %, which is below that of the
aluminum-alumina-basedmolded products obtained in Example 1. Such a small change is
considered to be due to the effect of the grain size distribution. Generally, when
the sintering temperature increases the sintered product shrinks to a greater extent.
Sintering at 1537K shows an expansion of 0.4% for about 40 vol.% aluminum. The reduction
and oxidation reaction (thermit reaction) between silica and aluminum in mulite accelerates
the internal oxidation of aluminum to form an alumina + silicon composite.
Example 3
[0025] Aluminum grains having an average grain diameter of 20 µm were used for metal grains,
and alumina grains (average grain diameter of two levels: 4 and 44 µm) were used for
ceramic grains in various compounding ratios, and both were kneaded. Then hydrolyzed
ethyl silicate was added as the binder to form a slurry. The slurry was kneaded by
vibrations and was subjected to vacuum degassing followed by casting in an aluminum
mold to form a specimen of & 10 x 20mm. After the alcohol was removed by air drying,
sintering was conducted in a nitrogen atmosphere for 6 hours. After X-ray diffraction
and determining the mechanical properties of the sintered aluminum nitride product
the sintering behavior of the sintered aluminum nitride product was examined. As in
the experiment of direct oxidization in Example 1, alumina grains having two levels
of grain-diameter distribution relative to aluminum grains were used. The conventionally
reported sweating phenomenon of aluminum was not observed for the aluminum grains
of a low amount, such as about 20% of aluminum, but was observed only for the aluminum
grain of a high amount, such as 78% or more aluminum. It is considered that, since
the specimen in this experiment is a porous product obtained by non-pressure molding,
sufficient spaces among the ceramic grains were present. When the average diameter
of the ceramic grains was below that of aluminum grains, the dimensional shrinkage
was 4%. As shown in Figure 13, the strengths were substantially constant. When the
average grain diameter of ceramic grains was above that of the aluminum grains, the
strengths increased linearly. In contrast, when the content of the aluminum was 50
wt.% or more, shrinkage was significant, and the sintering behavior was different
from that in the oxidation sintering of aluminum. It is considered that this behavior
was caused by the nitriding reaction of aluminum among ceramic grains on the surface
of the specimen. As shown in Figure 14, it was confirmed that the sintering of the
aluminum nitride composite material showing dimensional expansion was caused by the
nitriding sintering of the aluminum-alumina grains.
[0026] As explained above, prominent effects, such as a ceramic porous body with good porosity
and mechanical properties being prepared at a low cost by simple processes, are obtained
by the present invention.
Example 4
[0027] Aluminum grains having an average grain diameter of 15 µm were used for the metal
grains and alumina grains having a broad grain diameter distribution and an average
grain diameter of 38 µm were used for the ceramic grains in various compounding ratios,
and both were kneaded. Then hydrolyzed ethyl silicate was added as the binder to form
a slurry, the thus-obtained slurry was kneaded by vibrations and the kneaded product
was subjected to vacuum degassing. The thus-obtained product was cast in an aluminum
mold to form a specimen of & 10x20mm. After the specimen was removed from the mold
and then subjected to air drying, it was sintered in the atmosphere at 1573K. When
the specimen was formed by adding 37% by volume of aluminum, the oxidation behavior
of the specimen upon sintering became uniform. Thus a 37 vol.% aluminum-containing
slurry was cast in three kinds of aluminum molds to form specimens of & 10x20, $ 20x43,
and & 43x53mm. Even though the same slurry was used, a density change depending on
the difference among specimen sizes (mass effect) was observed. Thus, the effect of
using different amounts of a binder was examined using the same 37% aluminum-containing
specimens. As shown in Fig. 15, in all of the specimens, as the amount of the binder
increased the specific gravity tended to get lower. However, significant changes occurred
for molded products in which 26% by weight of the binder was contained. That is, although
portions of the same slurry were used for molded products, the densities of the products
were different. This shows that the slurry in this batch was nonuniform.
[0028] When, to resolve this problem, alumina fibers (manufactured by Nichias, 4) 3x100
µm) were compounded, as shown in Fig. 16, the variations in density of the molded
products decreased. Although the tendency of nonuniformity in the case of the aforementioned
26% binder is most significant, the addition of 4% by volume or more of the fibers
makes the variation settle in a certain range. As the amount of added fibers increases
the compressive strength also increases and the variation in porosities decreases.
Brief Description of Drawings
[0029]
Fig. 1 is an enlarged view of a green molded product.
Fig. 2 is an enlarged view of a product of a bonded structure consisting of metal
grains that are not chemically combined with gas and added ceramic grains in a ceramic
porous body obtained by sintering the molded product in a gaseous atmosphere.
Fig. 3 is an enlarged view of a product of a bonded structure consisting of partially
gas-reacted metal compound grains, with unreacted metal grains remaining, and added
ceramic grains bonded to said compound grains, in a ceramic porous product obtained
by sintering the molded product in a gaseous atmosphere.
Fig. 4 is an enlarged view of a product of a bonded structure consisting of gas-reacted
metal compound grains completely sintered, and added ceramic grains, in a ceramic
porous product obtained by sintering the molded product in a gaseous atmosphere.
Fig. 5 is an enlarged view of a product of a bonded structure consisting of metal
compound grains that are chemically combined with gas, metal compound grains that
are chemically combined with a decomposition gas of added ceramic grains, added ceramic
grains, and a metal produced by the decomposition of added ceramic grains, in a ceramic
porous body obtained by sintering in a gaseous atmosphere.
Figs. 6 and 7 are sectional views that schematically show a production process for
preparing a ceramic porous body obtained by the present invention.
Fig. 8 is a graph that shows the amount of a binder necessary for forming a slurry
of a mixture of alumium grains and an alumina having a different average grain size.
In this graph A and A show the data obtained by using alumina A-12 as the ceramic
grains, while 0 and C show the data obtained by using alumina A42-6 as the ceramic
grains.
Fig. 9 is a graph that shows the compressive strengths and the dimensional changes
of a molded aluminum-alumina product obtained by sintering the molded product in a
gaseous atmosphere.
Fig. 10 is a graph that shows the amount of a binder necessary for forming a slurry
of a mixture of aluminum or an aluminum alloy and mulite. In this graph - shows the
data obtained by aluminum as the metal grains, while 0 shows the data obtained by aluminum-12% silicon as the metal grains.
Fig. 11 is a graph that shows the compressive strengths of an aluminum/mulite molded
product obtained by sintering the molded product in the atmosphere.
Fig. 12 is a graph that shows the dimensional changes of a molded aluminum/mulite
product obtained by sintering the molded product in the nitrogen atmosphere.
Fig. 13 is a graph that shows the compressive strengths of an aluminum/alumina molded
product obtained by sintering the molded product in a nitrogen atmosphere.
Fig. 14 is a graph that shows the dimensional changes of an aluminum/alumina molded
product obtained by sintering in a nitrogen atmosphere.
Fig. 15 is a graph that shows the effect of a binder on the density of an alumina-37%
aluminum-molded product.
Fig. 16 is a graph that shows the effect of fibers on the density of an alumina-37%
aluminum- molded product.
[0030] In those graphs fine grain means the case where an alumina having an average grain
diameter of 4 a was used and coarse grain means the case where an alumina having an
average grain diameter of 44 a was used.